Field of an Electric Charge

In middle school you mapped magnetic field lines using iron filings.

In class, we saw a demonstration of electric field lines using grass seeds suspended in mineral oil. When the grass seeds are exposed to a strong electric field, the grass seeds become polarized and line up along the field lines. There is a video of this.

The one geometry which is easy to calculate is for the field due to a point source, or a large distance from a small spherical source.

In this case, we can use E = F/q and substitute in Coulomb’s Law,

F=kQq/d2

where Q is the charge on the object in question, and q is our test charge. In this case, the test charge cancels and the field strength E, a distance d from from a charge Q is

E=kQ/d2

You will find a couple of workbook equations using this. What is interesting to remember is that Electric Field is a vector. If you wanted to find the field strength due to multiple charges, you would calculate each field separately and then add up the fields (remember it is a vector!).

Also, if you were to fire an electron parallel to a field, the field would accelerate the electron.

However, if you fired an electron perpendicular to a field, the field would exert a force perpendicular to the original motion, and it would follow a parabola. You could think of this as a form of projectile motion.